FIG. 1 illustrates the cross-section of the conventional "Double D" dual lumen catheter 10 of the type described in U.S. Pat. No. 4,134,402, Des. 272,651, U.S. Pat. Nos. 4,568,329, 4,692,141, 4,583,968, 4,770,652, 4,808,155, 4,842,582, 4,895,561, and Canadian Patent No. 1,193,508 by Dr. Sakharam Mahurkar for use during hemodialysis to prevent the need to insert separate catheters for extracting the patient's untreated blood and returning the treated blood to the patient. As known to those skilled in the art, the catheter 10 is inserted into a blood vessel (vein and/or artery) of the patient, and the patient's untreated blood is extracted through lumen 12 for extracorporeal blood purification by a hemodialysis device. The dialyzed blood is then returned to the patient's blood vessel via lumen 14. Septum 16 separates lumens 12 and 14 and divides the catheter 10 into two equal halves whereby the lumens hold substantially similar volumes of blood. Septum 16 further prevents the intermixing of the treated and untreated blood. During hemodialysis, such a geometry is purported to provide a bidirectional blood flow rate of 250 ml per minute with a pressure gradient under 100 mm Hg.
FIG. 2 illustrates the cross-section of the conventional "Circle C" dual lumen catheter 18 of the type described, for example, in U.S. Pat. No. 5,380,276 to Miller et al. The catheter 18 disclosed by Miller et al. operates in a similar manner as that illustrated in FIG. 1 except for the cross-sectional configuration of the catheter. As shown in FIG. 2, the "venous" lumen 20 for returning the treated blood to the patient has a circular cross-section, while the "arterial" lumen 22 for removing the untreated blood from the patient has a crescent-shaped cross-section. Miller et al. specifically designed the catheter of FIG. 2 so that the cross-sectional area of venous lumen 20 is substantially equal to the cross-sectional area of arterial lumen 22 for each catheter french size. Circular septum 24 prevents the intermixing of the treated and untreated blood.
Catheters with lumens having cross-sections with sharp corners as in FIGS. 1 and 2 have been found to cause platelet formation in the corners due to the reduced blood flows in the corners. Accordingly, the present assignee previously designed a modified "Circle C" dual lumen catheter 26 having all smooth transitions within the cross-sections of the arterial lumen 28 and the venous lumen 30 by using a septum 32 which was thickened in the corners 34 as illustrated in FIG. 3. Catheter 26 was also designed to have unequal cross-sectional areas for the respective lumens. In particular, the arterial intake lumen 28 was designed to have a cross-sectional area substantially larger than the cross-sectional area of the venous return lumen 30. Such a design was found to lower the intake vacuum and to raise the venous pressure to a substantially balanced level for each flow rate.
When treating "critical care" patients subject to renal failure, it is often desired to provide additional lumens besides the venous and arterial lumens in order to facilitate the insertion of a guide wire or therapeutic agents, to monitor hydrostatic pressure of the patient, and the like. Triple lumen catheters have been designed for this purpose. Unfortunately, not all provide cross-sections which permit flow rates sufficient for hemodialysis.
FIG. 4 is a cross-sectional view of a prior art low flow triple lumen critical care catheter 36 designed by the present assignee. In this design, three independent dedicated lumens 38, 40, and 42 are provided in a single catheter 36 for allowing the simultaneous and continuous monitoring and treatment of several parameters of seriously ill patients. However, because of the small size of the lumens relative to the outer diameter of catheter 36 and the resulting inefficient flow geometries, the design of FIG. 4 has not been found to be particularly suitable for critical care applications such as hemodialysis which require high flow rates on the order of 250 ml per minute.
Prior art triple lumen "critical care" catheters designed for higher flow rates are illustrated in FIGS. 5-9. For example, FIG. 5 is a cross-sectional view of a prior art triple lumen critical care catheter 44 of the type disclosed by Mahurkar in U.S. Pat. No. 5,221,256. As shown in FIG. 5, Mahurkar substantially maintained the "Double D" design for the venous lumen 46 and the arterial lumen 48 except that a small third lumen 50 for accepting a guide wire is located at the intersection of one diametral end of the septum 52 and the outer tube 54 between a pair of adjacent corners of the lumens 46 and 48. As in the "Double D" dual lumen configuration, the cross-sectional areas of the venous lumen 46 and the arterial lumen 48 are kept balanced.
FIG. 6 is a cross-sectional view of a prior art triple lumen critical care catheter 56 of the type disclosed by Young in U.S. Pat. No. 5,451,206 for maximizing the total cross-sectional area of the combined lumens. In particular, catheter 56 includes a major septum 58 which divides the catheter generally into venous lumen 60 and arterial lumen 62. However, septum 58 bifurcates into a pair of generally flat minor septums 64 and 66 to form a triangular-shaped third lumen 68 located between the adjacent corners of the generally D-shaped lumens 60 and 62. As in the triple lumen catheter illustrated in FIG. 5, the venous lumen 60 and arterial lumen 62 of catheter 56 of FIG. 6 have substantially the same cross-sectional areas and sufficient size to support flow rates of at least 250 ml per minute. However, as noted above, catheters with lumens having cross-sections with such sharp corners have been found to cause platelet formation in the corners due to the reduced blood flows in the corners and are not particularly desirable for hemodialysis applications.
FIGS. 7 and 8 illustrate triple lumen versions of the "Circle C" design described above with respect to FIG. 2 but tailored to the needs of patients suffering from shock, trauma, dehydration, or chemical or infectious circulatory collapse. In particular, FIGS. 7 and 8 are cross-sectional views of prior art triple lumen critical care catheters of the type disclosed by Mahurkar in U.S. Pat. No. 5,378,230. In FIG. 7, the large lumen 72 of catheter 70 is circular in cross-section and is used to infuse large volumes of liquid into the patient at a high rate, as when fluids are infused from an I.V. fluid source hung from a height of three to four feet. In other words, large lumen 72 functions as a venous lumen for infusing fluids into the patient. Septum 74 separates the crescent shaped lumen into second and third lumens 76 and 78 bounded by the outer diameter 80 of the catheter 70 for injections of medications, for taking blood samples, or for monitoring the hydrostatic pressure in the blood vessels of the patient. FIG. 8 illustrates a closely related embodiment of a catheter 82 in which the large venous lumen 84 is elliptical in cross-section and the septum 86 separates the crescent shaped lumen into relatively smaller second and third lumens 88 and 90 bounded by the outer diameter 92 of the catheter 82. As in the embodiment of FIG. 7, lumens 88 and 90 are used to inject medications, to take blood samples, or to monitor the hydrostatic pressure in the blood vessels of the patient. Mahurkar does not suggest in U.S. Pat. No. 5,378,230 that such "unbalanced" catheters may be used for hemodialysis applications.
FIG. 9 is a cross-sectional view of a prior art "longhorn" triple lumen critical care catheter 94 previously designed by the present assignee to provide flow rates suitable for hemodialysis. The "longhorn" triple lumen design includes venous lumen 96 and arterial lumen 98 separated by a planar septum 100. The third lumen 102 is located at the intersection of one diametral end of the septum 100 and the outer tube 104 between a pair of adjacent corners of the lumens 96 and 98. As in the configuration of FIG. 5, the cross-sectional areas of the venous lumen 96 and the arterial lumen 98 are kept balanced. A triple lumen critical care catheter which supports flow rates suitable for hemodialysis applications and performs as well or better than the designs of prior art FIGS. 5 and 9 is desired. As will be explained in more detail below, the present inventor has discovered that improved flow rates could be achieved by providing "unbalanced" smooth arterial and venous lumens with smooth contours. Substantial improvements in flow rates and pressures for a given french size were discovered for the particular configurations described below.